The present invention relates to a process for the preparation of a lithium manganese oxide by reacting at least one lithium compound and at least one manganese compound under hydrothermal conditions, that is to say from heated aqueous solutions under subatmospheric pressures, to a lithium manganese oxide of this type per se and to the use thereof as cathode material for electrochemical cells.
Processes for the preparation of lithium manganese oxides of the approximate composition LiMn2O4 are known per se.
Thus, U.S. Pat. No. 4,980,251 describes a process for the preparation of a lithium manganese oxide of the formula Li1xe2x88x92xMn2O4 with 0xe2x89xa6x less than 1 by a solid phase reaction in which the corresponding lithium manganese oxide is obtained by mixing the corresponding oxides and/or carbonates together and heating the resulting mixtures in an oxidizing atmosphere to a temperature in the range from 200 to 600xc2x0 C. This results, as stated in U.S. Pat. No. 4,980,251, in spinels with exceptionally high BET surface areas of, for example, more than 30 m2/g with, at the same time, poor crystallinity. Spinels of this type are said according to the publication to be particularly suitable for use in electrochemical cells.
DE-A-43 27 760 relates to a process for the preparation of a positive electrode for lithium secondary batteries whose active material consists of a lithium manganese oxide which has a spinel structure and which is obtained by mixing manganese dioxide with lithium formate and/or lithium acetate and subsequently heating the mixture at a temperature of from 550 to 800xc2x0 C., followed by grinding.
Another process for the preparation of such oxides with a spinel structure is claimed in U.S. Pat. No. 5,135,732. In this case, firstly a gelatinous precipitate is formed by mixing stoichiometric amounts of an aqueous solution of lithium hydroxide and manganese acetate in the presence of a base at a pH of approximately 7. This precipitate is then dried at 60 to 150xc2x0 C. to form a xerogel. Finally, the xerogel is heated to a temperature of from 200 to 600xc2x0 C., resulting in a lithium manganese spinel.
DE-A 195 19 044 describes lithium- and manganese(III/IV)-containing spinels with a specific surface area of from 0.1 to 4 m2/g, and the use thereof as cathode material for electrochemical cells. The process indicated for the preparation of the spinels claimed therein is reaction of the starting compounds at elevated temperature.
However, the abovementioned processes have disadvantages with regard to the homogeneity of the resulting products. Furthermore, these processes often result in by-products which are unusable or usable only with difficulty, such as, for example, salts and off-gases.
Although the prior art publications discussed above also disclose the use of the oxides prepared therein as cathode material in electrochemical cells, the oxides with a spinet structure prepared by the prior art processes show a comparatively poor and often inadequate cycle stability on use as cathode material in electrochemical cells.
The use of LiCoO2 and LiNiO2 as cathode materials for electrochemical cells is the state of the art. Because of the relatively high material costs, the limited availability of cobalt, the environmental objections thereto and a potential danger associated with use of these compounds on overcharging, there is great interest in replacing the abovementioned compounds by, for example, lithium manganese oxides with a spinel structure, as also proposed in the prior art publications quoted above. Thus, for example, D. Fouchard et al. describe, in a scientific paper in xe2x80x9cThe Electrochemical Society Proceedingsxe2x80x9d, vol. 94-28, page 348, the use of lithium manganese oxides with a spinet structure in electrochemical cells.
In view of the above prior art, the object of the present invention is to provide a process which, on the one hand, makes it possible to prepare lithium manganese oxides, in particular inter alia pure-phase lithium manganese oxides with a spinet structure, and which furthermore can be carried out on the industrial scale without difficulty and with avoidance of by-products which cannot be utilized or are even hazardous, and results in lithium manganese oxides with very good electrochemical properties.
This object is achieved by a process for the preparation of a lithium manganese oxide of the formula Li1.5xe2x88x92xMn2O4 where the value of x satisfies the relation 0xe2x89xa6x less than 1.5, which comprises the following step: reaction of at least one lithium compound and at least one manganese compound, characterized in that the reaction is carried out in an aqueous medium at a temperature in the range from 80 to 500xc2x0 C. under a pressure of from 1xc3x97105 Pa to 5xc3x97107 Pa.
As is evident from what has been said above, it is possible in the process according to the invention to obtain not only lithium manganese oxides with a spinel structure and the approximate composition LiMn2O4, but also oxides with a very small content of lithium, including xcex-MnO2. However, an oxide with a spinel structure is preferably obtained.
Although it is possible in principle to employ all lithium compounds in the process according to the invention, those preferably employed are Li2O, LiOH, LiCl, LiNO3, Li2CO3, Li carboxylates such as, for example, Li formates or Li acetate, or a mixture of two or more thereof.
There are also in principle no restrictions concerning the manganese compounds which can be employed in the process according to the invention. However, the manganese compound preferably employed is MnO2, MnO, MnOOH, Mn2O3, Mn3O4, MnCO3, Mn(NO3)2, Mn carboxylates such as, for example, Mn formate or Mn acetate or a mixture of two or more thereof, those employed being in particular oxidic manganese raw materials and manganese mixed oxides such as, for example, MnO, MnOOH, Mn2O3, Mn3O4 and MnCO3, and mixtures of two or more thereof.
It is also possible furthermore for the lithium manganese oxides prepared according to the invention to contain in addition another metal M or a mixture of two or more other metals, preferably a metal of group IIa, IIIa, IVa, IIb, IIIb, IVb, VIb, VIIb or VIII of the Periodic Table, or a mixture of two or more thereof, and, in particular, iron, cobalt, nickel, titanium, boron, aluminium or a mixtureof two or more therof, resulting in oxides of the general formula
xe2x80x83Li1.5xe2x88x92xMzMn2xe2x88x92zO4,
where x is as defined above, and the value of z satisfies the relation 0.01xe2x89xa6zxe2x89xa61.
Metal-doped Lixe2x80x94Mn-oxides of this type are prepared by employing in the reaction according to the invention a salt of a metal or a mixture of two or more thereof, as defined above, preferably an Fe, Co, Ni salt or a mixture of two or more thereof, in each case in the required amount.
Accordingly, the present invention also relates to a process for the preparation of a lithium manganese oxide of the formula Li1.5xe2x88x92xMzMn2xe2x88x92zO4 where the value of x satisfies the relation 0xe2x89xa6xxe2x89xa61.5, M is a metal of group IIa, IIIa, IVa, IIb, IIIb, IVb, VIb, VIIb or VIII of the Periodic Table, or a mixture of two or more thereof, and the value of z satisfies the relation 0.01xe2x89xa6zxe2x89xa61, preferably the relation 0.1xe2x89xa6zxe2x89xa60.4, which comprises the following stage: reaction of at least one lithium compound, at least one manganese compound and at least one compound of a metal as defined above, characterized in that the reaction is carried out in an aqueous medium at a temperature in the range from 80 to 500xc2x0 C. under a pressure of from 1xc3x97105 Pa to 5xc3x97107 Pa.
The lithium compound and manganese compound are preferably employed in an amount such that the manganese:lithium molar ratio is 2:approximately 1, although Mn:Li molar ratios of from 2:approximately 1.5 to approximately 2.5, preferably 2:approximately 1.3 to approximately 0.6, can also be chosen.
The special feature of the process according to the invention is to be regarded as being the reaction of the lithium compound and manganese compound under hydrothermal conditions. This means that the reaction takes place in an aqueous medium. The reaction takes place at a temperature in the range from approximately 80 to approximately 500xc2x0 C., preferably approximately 100 to approximately 350xc2x0 C. and, in particular, approximately 150 to approximately 250xc2x0 C.
The reaction is carried out under a pressure of approximately 1xc3x97105 Pa to approximately 5xc3x97107 Pa, preferably approximately 1xc3x97106 Pa to approximately 5xc3x97106 Pa and, in particular, under approximately 1.5xc3x97106 Pa to approximately 2.5xc3x97106 Pa.
Depending on the components employed, the actual reaction can be followed by a washing process to remove dissolved impurities present in the metal salts employed, such as, for example, sulphates, Na and K salts, and other components which are not a constituent of the spinel according to the invention.
The product obtained in the reaction in the present process can additionally be dried, the temperatures used for this preferably being in a range from approximately 60 to approximately 200xc2x0 C., furthermore preferably approximately 100 to approximately 150xc2x0 C. and, in particular, approximately 120 to approximately 135xc2x0 C.
It is furthermore possible for the reaction or for the reaction and drying to be followed by the product obtained thereby being additionally heat-treated, this preferably being done at a temperature in the range from approximately 200 to approximately 900xc2x0 C., furthermore preferably approximately 300 to approximately 850xc2x0 C. and, in particular, at approximately 800xc2x0 C.
The duration of the heat treatment is preferably at least approximately one hour, furthermore preferably more than approximately 15 hours and, in particular, approximately 24 hours, with the maximum duration of the heat treatment being approximately 30 hours, especially for economic reasons.
It is particularly suitable to carry out a heat-treatment stage when the manganese valency is, for example, above approximately 3.5. During this heat-treatment process, which influences the oxygen content and the Mn valency, the valency of manganese is optimized towards 3.5. In this connection, the valency indicates the average oxidation state of the manganese present in the oxide.
It is possible to reduce the Li content when employing of an elevated amount of Li, or to reduce this content generally after the actual reaction by means of an acid-leaching process which can preferably take place before the optional drying or heat treatment. It should be noted in this connection that the aqueous phase can be removed before the drying, although it is generally possible to dispense with this, for example in the case of spray drying.
The crystallinity and the specific surface area of the lithium manganese oxide obtained by the process according to the invention can be influenced by specifically controlling the temperature, pressure and reaction time during the reaction and/or the subsequent heat treatment.
The particle size of the lithium manganese oxide obtained according to the invention can be influenced by taking into account of the particle fineness of the manganese raw materials employed by preceding wet or dry milling processes. It is likewise possible to reduce the particle size by grinding the wet or elsewise dried lithium manganese oxide. The spinel obtained according to the invention preferably has an average particle size, measured using a Cilas granulometer, in the range from approximately 0.5 to approximately 100 xcexcm, furthermore preferably from approximately 1 to approximately 50 xcexcm.
The BET surface area of the lithium manganese oxide prepared according to the invention is preferably below approximately 10 m2/g, furthermore preferably below approximately 8 m2/g and, in particular, approximately 5 m2/g, the lower limit being at approximately 0.1 m2/g.
It is possible in the present process to carry out successfully in particular the following reaction variants:
The present invention furthermore also relates to a lithium manganese oxide of the formula Li1.5xe2x88x92xMn2O4 or a lithium manganese oxide of the formula Li1.5xe2x88x92xMzMn2xe2x88x92zO4, obtainable by reacting at least one lithium compound and at least one manganese compound or at least one lithium compound, at least one manganese compound and at least one compound of a metal M, characterized in that the reaction is carried out in an aqueous medium at a temperature in the range from 80 to 500xc2x0 C. under a pressure of from 1xc3x97105 Pa to 5xc3x97107 Pa.
This oxide has, of course, the same properties concering the BET surface area,the Li content and Mn content and the particle size as stated in the description of the process according to the invention.
The lithium manganese oxide described above is preferably one with a spinel structure, in particular a pure-phase Li Mn oxide with a spinel structure.
The lithium manganese oxide according to the invention or prepared according to the invention, preferably with a spinel structure, is a so-called intercalation compound. Compounds of this type are able to store active Li+ ions in a host lattice, the lithium being embedded in interstitial sites in the host material. Accordingly, intercalation compounds of this type are particularly useful for electrochemical cells. Entry into and exit from interstices can be effected electrochemically with high reversibility by means of an electrolyte conducting lithium ions, preferably LiClO4, LiBF4, LiPF6, LiSO3CH3, LiAsF6. Combination of two different intercalation compounds with very different electrochemical potentials results in an accumulator in which lithium ions migrate forwards and backwards between the host materials during the charging and discharging processes. Electrochemical cells of this type are referred to as lithium ion cells. When a lithium manganese oxide, preferably with a spinel structure, is used, a suitable host material for the anode is, in particular, a carbon-containing substance such as, for example, graphite or coke. A cell of this type might be referred to briefly as corresponding to the LixC6/Liyxe2x88x92xMn2O4 type.
Accordingly, the present invention also relates to the use of the lithium manganese oxide according to the invention or prepared according to the invention, preferably with a spinel structure, as cathode material or as part thereof for an electrochemical cell. The lithium manganese oxide in this case represents, where appropriate in conjunction with a binder such as, for example, PTFE and acetylene black, the cathode, while the anode preferably has as host material a carbon-containing substance as defined above, or metallic lithium.
There are in principle no special restrictions on the choice of the anode on use of the lithium manganese oxide described herein as cathode. The compounds which can be used merely need to allow intercalation of lithium ions and have a higher electrochemical activity than the cathode.
The lithium manganese oxide according to the invention or prepared according to the invention, preferably with a spinel structure, is prepared as battery cathode material in a manner known per se. This cathode material can be employed in a manner known per se in an electrochemical cell having an anode which receives lithium cations. The electrodes in such cells in the fully assembled and closed state are usually uncharged, that is to say all the available lithium is embedded in the positive electrode, while the host structure of the negative electrode is in a lithium-unloaded state. On initial charging, the lithium exits from the positive host lattice (cathode) and enters into the negative host lattice (anode), preferably a carbon matrix. Some of the lithium ions which are irreversibly attached to the carbon matrix and are unavailable for the subsequent intercalation mechanism can be compensated by an above-stoichiometric amount of lithium in the lithium manganese oxide, which preferably has a spinel structure.
The design of such electrochemical cells in principle is known and is described inter alia by J. M. Tarascon in J. Electrochem. Soc. 140, page 3071 ff.
On use as battery cathode material the lithium manganese oxides according to the invention or prepared according to the invention, preferably with a spinel structure, show good capacitive properties and cycle stability, as is proved by the examples.
Accordingly, the present invention also relates to an electrochemical cell which has at least one cathode which contains the lithium manganese oxide according to the invention or prepared according to the invention, preferably with a spinel structure.